The textbook appears to go back and forth between the systematic name and common name for both primary and secondary/tertiary amines. However, it seems to use the common name for amines that have multiple of the same substituent. Is this a preference or an indication that both forms can be used interchangeably?

explanation:

venn diagram is pictorial representation with of circles showing the relation between two finite sets or things. the intersection part of two circles conveys the relation present in between those sets or things.

here in the venn diagram of sand and salt two circles represent sand and salt. but the area falling under the intersection of these circle represents the mixture of sand and salt.

so, in a venn diagram the mixture will be placed in a middle that is in between the intersection of two circles.